Solders are used in low temperature, usually reversible, metallurgical joining processes. Low temperature solders with reversibility are especially important in electronic applications. The low temperature is required since many materials are damaged by even moderately high temperatures. The reversibility is required since reworking of products is often necessary. Low temperature soldering is extremely well suited for this.
Solder joining is a wetting process followed by a chemical reaction. Molten solder wets the substrate selectively. The selective wettability of the solder allows molten solder to be confined to metallic pads and not to solder mask materials. This is especially important in flip Chip bonding and surface mount attachment of Components such as quad flat packs, and ball grid array modules.
The soldering process takes place virtually as quickly as the wetting process once the solder has melted. For example, with rapid heating, soldering can take place in just a few seconds. This makes soldering particularly desirable for automated, high-speed, high through-put processes.
Wettability is not only a function of the solder material, but is also a function of the materials to be joined by the solder, such as copper, nickel, gold and palladium, as well as those rich in one or more of these metals which are particularly amenable to soldering.
Chemical reaction following wetting occurs between the liquid solder and the materials being joined, which forms intermetallic phases at the interfaces. The intermetallic phases formed by solders in electronic packaging are stoichiometric compounds, typically binary compounds and typically containing tin if tin is present in the solder alloy. If one of the metals to be joined is copper and the solder alloy in rich in tin, the intermetallic compound formed during soldering is Cu--Sn. Cu--Sn binaries include Cu.sub.3 Sn and CU.sub.6 Sn.sub.5, although other intermetallics may be formed.
Solder alloys are characterized by the melting temperature being a function of composition. Thus, while a pure metal is characterized by a single invariant melting temperature, the freezing and melting points of alloys are complex. The freezing point of an alloy is determined by the liquidus line, wherein only a liquid phase exists above the liquidus line. The melting point of an alloy is determined by the solidus line, wherein only a solid phase or phases can exist below the solidus line. In the region between the solidus and liquidus lines, solid and liquid phases generally co-exist. Many soldering alloys are eutectic; i.e., they are characterized by a eutectic point. The eutectic point is where the liquidus and solidus lines meet, and thus there is a single melting temperature representing both the liquidus and solidus temperature. A change in concentration of the element in either direction from the eutectic composition results in an increase in the liquidus temperature, and also generally in a separation between the liquidus and solidus lines, with liquid and solid phases therebetween as indicated above. The composition and quench rate also determine the microstructure and resulting mechanical properties of a solder joint. Thus, it is necessary to both carefully choose the solder composition and to control the thermal exposure and thermal excursions of the solder joint.
One very common type of solder composition used in electronics fabrication is the tin/lead alloys. These alloys are capable of forming electrically-conductive, thermally stable, non-brittle intermetallics with the material being joined. One particular alloy that is well known is a eutectic tin/lead composition which contains about 63% tin and 37% lead. This particular alloy, being a eutectic, has a melting point of about 163.degree. C. (compared to Sn which has a melting point of 232.degree. C. and Pb which has a melting point of 327.degree. C.). This low melting point, plus the workability of the lead/tin alloys and the adhesion of the copper/tin intermetallics over a wide temperature range, and the availability of equipment and related materials for the process has made the tin/lead alloys extremely desirable. This relatively low temperature in non-damaging to most electronic components and other materials such as organic substrates, and the process is reversible.
Another important characteristic of this material is the softness or plasticity of these lead-base solders. This softness or plasticity allows the solder to accommodate the mismatch in coefficients of thermal expansion of bonded structures. For example, the mismatch in coefficients of thermal expansion between a ceramic dielectric and a polymeric dielectric, or between a semiconductor chip and a ceramic or polymeric chip carrier or substrate, can readily be accommodated.
However, one major drawback to the tin/lead alloys is that lead is toxic and has a relatively high vapor pressure. Thus, while in many cases there is not a large amount of lead present, nevertheless the accumulation of lead, even in small amounts, can be unacceptable and thus the use of lead is becoming more and more disfavored, with a replacement for the lead being required.
U.S. Pat. No. 5,256,370 to Slattery, et al. suggests certain solder alloys containing Sn, Ag, and In where Ag is specifically limited to less than 6% by weight. Also, U.S. Pat. No. 5,328,660 to Gonya, et al., suggests a quaternary solder alloy of 78% Sn, 2% Ag, 9.8% Bi, and 9.8% In. U.S. Pat. Nos. 4,998,342 and 4,761,881 suggest pin-in-hole and surface mount assemblies using wave soldering and solder paste. These patents are incorporated herein in whole by reference.